Positron emission tomography (PET) is an imaging technique that is frequently used in nuclear medicine. In PET, a physiologically relevant compound is labelled with a positron-emitting isotope. At some point in time, the radioactive isotope emits a positron, and this positron has a kinetic energy of a few hundreds of keV. The range of the positron in human tissue is usually less than 1 mm. After coming to rest, the positron annihilates into two nearly back-to-back gamma rays of 511 keV. The mean free path of gamma rays of 511 keV in the human body is about 10 cm. In many cases, the two gamma rays will leave the body without undergoing scattering, i.e. with their original direction unchanged.
A PET scanner basically is a detector that surrounds a patient for detecting for example gamma rays of 511 keV. If two 511 keV gamma rays are detected at the same time, these most likely come from the same annihilation event. It can therefore be assumed that the annihilation, and the molecule containing the radioactive isotope, was somewhere on the line joining the two detection points. This line is called a “line of response” (LOR). From observing a large number of positron annihilations in this way, it is possible to derive a three-dimensional distribution of the annihilation events, which is the same as the three-dimensional distribution in the body of the labelled molecules.
Commercial PET scanners typically use scintillating crystals and photodetectors to detect the gamma rays. When a gamma ray interacts in the scintillating crystal, a brief and weak light signal is generated. The light emission can be in the visible range of the optical spectrum, in the ultraviolet or the infrared. Commonly used scintillator materials in PET are BGO (Bi4Ge3O12), LSO(Lu2SiO5:Ce); LYSO(Lu2-2xY2xSiO5:Ce); GSO (Gd2SiO5:Ce), and NaI:Tl. The most important properties of a scintillator to be used in a PET scanner are that it must have a short decay time, have a large stopping power for X and gamma rays, and a good energy resolution. The short decay time is important because this allows a good time resolution, and this ensures that the two gamma rays that are detected really come from the same annihilation event rather than from two unrelated annihilation events. The stopping power is important because this ensures a large detection efficiency, and therefore that a large fraction of the annihilation events will be observed. The good energy resolution allows rejection of events where one of the gamma rays undergoes Compton scattering before it is detected.
Although commercially available PET scanners are capable of providing a three-dimensional image of processes in the human body, they do not always provide as accurate an image as desired.